The present invention relates to improvements in or relating to loudspeakers and more particularly, though not exclusively, to .an improved chassis for a loudspeaker.
A loudspeaker is a device for converting variations in electrical energy into corresponding variations of acoustic energy or sound. A loudspeaker comprises a permanent magnet whose field acts on a current carrying conductor causing it to move at right angles to the lines of magnetic force. The conductor is coupled to a resiliently mounted diaphragm which causes it to move such that the diaphragm vibrates in relation to the current variations and transmits these vibrations to the air as sound waves.
In moving coil loudspeakers the armature which vibrates in the magnetic field comprises a coil attached to a conical diaphragm. In such a speaker the moving coil oscillates inside an electromagnet which is energised to a direct current.
In a permanent magnet moving coil speaker the coil oscillates in the annular cavity of a specially shaped permanent magnet and it is for this type of speaker which the invention has particular application, although it is to be understood that the invention is in no way limited to such a speaker type.
A typical moving coil loudspeaker employs a magnet and a chassis (the hardware) along with the coil, diaphragm and suspension system (the software). The magnet is heavy and needs to be supported by the chassis which also ensures alignment of the software relative to the voice coil gap. This alignment is essential to allow free movement of the coil within the voice coil gap.
Traditionally chassis are either metal pressings, metal castings or plastic mouldings, and these are placed behind the cone or dome of a speaker.
A problem with moving coil speakers is that when they are used at high powers, the voice coil heats up rapidly, resulting in an increased resistance, and a subsequent drop in power. Therefore, in order to allow for continued application at high power, without risk of damage to the components of the speaker, the heat produced must be dissipated. Due to the structure of speakers, the heat produced by the coil is currently dissipated by heat transfer to the magnet structure and chassis. Since the magnet structure and chassis are at the back of the speaker, and since speaker cabinets are usually lagged, the air inside the speaker is ultimately warmer resulting in a reduced efficiency of heat loss and a worsening of the situation.
For example, a voice coil with a dc resistance of 5 ohms at room temperature can, when driven at high power from an amplifier, have an effective d.c. resistance of 10 ohms. How much power the coil will accept before physical damage, i.e. breakdown of the insulating varnish on the wire or bonding adhesives, depends on the type of varnish and adhesive used. Since adhesives can run at much higher temperatures than those available thirty years ago, coils can withstand a greater power input and can be run at temperatures of around 200.degree.-250.degree. C. This increase in operating temperatures allowed by modern materials exacerbates the problem of increased voice coil resistance at high powers.
A coil will draw power from an amplifier according to the basic formula ##EQU1## Thus a doubling of resistance means that half power is drawn. Expressed logarithmically using decibels, this translates into a power compression of three decibels and is equivalent to a loss of speaker sensitivity of three decibels when the speaker in this example is run at high power levels. The change in voice coil resistance also modifies other speaker parameters affecting the performance in the bass frequencies.
Thus, in order to increase speaker power and improve speaker quality, it is necessary to reduce the heat increase of the coil.
The heat produced in the voice coil is lost to the magnet assembly and chassis by a process of radiation and conduction. Since the magnet and chassis are behind the cone assembly, the heat is transferred rapidly to the cabinet. The result is that the cabinet air becomes warmer resulting in a reduced efficiency of heat loss and as a result of acoustic wadding a lagging effect worsens the situation. As power to the speaker is increased there is an increase in acoustic output and an increase in voice coil temperature. This results in an increase in the d.c. resistance of the coil preventing it from drawing as much power as it would if it were cold. The power handling capacity of the speaker is thus set by the maximum allowable temperature of the voice coil in conjunction with the ability of the software to withstand the mechanical and thermal forces imposed on it. The usual solution to this problem is to increase a loudspeaker's thermal power handling capacity by the use of larger coils which increase the area from which heat can be lost to the surrounding metalwork.